Generally, roads, parking lots, or other areas that must carry heavy traffic comprise three elements: a subgrade, a base and a surface course. The surface course is in direct contact with the traffic, and the base transmits pressure exerted by vehicles on the surface course to the subgrade. Depending on strength requirements, different materials can be used to form the base; for example, granular materials such as crushed stone, sand, shell and other silicious or calcarious solids can be used.
Some high grade aggregate materials, when compacted correctly, form a road base with sufficient compressive strength so as not to require the addition of cement; these high grade aggregate materials are not always readily available, and are frequently more expensive than low grade aggregate materials mixed with cement. Since cement is substantially more expensive than either high or low grade aggregate materials, it is desirable to add as small a percentage of cement to an aggregate material as is necessary to achieve the required compressive strength. However, in areas where high grade aggregate materials are not available, and the only available aggregate materials are low grade aggregate materials that require large amounts of cement to form solids having sufficient compressive strength, road base construction is more expensive.
Due to the large volume and variety of materials processed by large municipal solid waste incineration facilities, the wastes are frequently not completely combusted, and the ash, particularly the fly ash, often contains dangerous concentrations of toxic elements such as cadmium and lead. To prevent continued combustion outside of the incinerator, water is sprayed on the burning wastes leaving the combustion zone; usually, large chunks of metal and uncombusted material remain in this wet bottom ash. Many facilities also combine the fly ash, which may contain toxic concentrations of heavy metals, with the wet bottom ash. The safe disposal of the wet, partially incinerated, and/or potentially toxic incinerator ash poses an expensive disposal problem for municipal solid waste incinerator facilities.
Coal fly ash is pozzolanic; a cementitious solid is formed by mixing coal fly ash with lime. For this reason, coal fly ash and lime have been used in place of cement, or in addition to cement, for road and building construction; coal fly ash is also known to increase the strength of concrete to which it is added to, and structures built using coal fly ash pose little danger to the environment as coal fly ash generally does not contain dangerous levels of toxic elements. If ash produced by municipal solid waste incinerator facilities could be used in a similar fashion to coal fly ash, a major environmental disposal problem could be solved, while simultaneously providing an inexpensive source of material that can be used in constructing roads and other structures.